The present invention relates to a method and apparatus for measuring by using microwaves the concentration of dissolving matter, such as pulp suspension liquid, in which suspended matter (e.g., pulp) and/or an additive is dissolved.
When microwaves are incident on water, the microwaves have a phase lag (.theta..sub.1) behind that in the case where microwaves are incident on a vacuum (air). When microwaves are incident on a suspension, such as sludge or pulp liquid, a greater phase lag (.theta..sub.2) results.
This phenomenon is represented by the following equation: EQU Concentration X=C.multidot.a.sub.k .multidot..increment..theta.=C.multidot.a.sub.k .multidot.(.theta..sub.2 -.theta..sub.1) (1)
where C: a correction coefficient, a.sub.k : a sensitivity coefficient, .theta..sub.1 : a phase lag in a case where microwaves are incident on reference liquid, and .theta..sub.2 : a phase lag in a case where microwaves are incident on suspension liquid.
The difference between the phase lags (phase difference: .theta..sub.2 -.theta..sub.1) is proportional to the sum of the concentration of suspended matter and the concentration of dissolved matter.
In recent years, concentration meters based on the above principle have been developed, and disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 4-238246 and 5-322801. The developed or disclosed concentration meters have many advantages, one of which is little influence of adhesion of suspended matter or bubbles in the liquid.
As regards pulp suspension liquid used in papermaker plants, a plurality of additives are added to produce each product (paper) in a constant mixture ratio. Assume that a pulp concentration (concentration of pulp as suspended matter) or additive concentration (concentration of an additive as dissolved matter) is measured by means of the concentration meter based on the aforementioned principle. When the concentrations of the additives (additives A and B) are individually measured, the indication of the concentration meter is proportional to the concentration, as shown in the equation (1) and FIG. 1. FIG. 1 is a diagram showing a result of measurement of individually measuring the pulp concentration and additive concentrations by a microwave concentration meter.
Since pulp and the additives A and B are different in physical properties, the sensitivity a.sub.k in the equation (1) is different between the additives. If the additives are mixed, the sensitivity of the mixture cannot be necessarily expected accurately, so that an error may arise in the measured concentration value depending on the mixture ratio. Further, in the case of the concentration meter based on the above principle, it is necessary to obtain in advance a reference phase lag (.theta..sub.1) using reference liquid, for example, service water, which has a concentration of substantially zero. This operation is called zero adjustment or zero-point adjustment.
To execute such a zero adjustment operation, it is necessary that a piping system including the concentration meter have a structure as shown in FIGS. 2A and 2B. Referring to FIGS. 2A and 2B, to perform zero adjustment, a stop valve 52 on the side of a bypass pipe 51 is first opened to allow pulp suspension to pass through the bypass pipe, and then, stop valves 53, 54 on the side of a main pipe 55 are closed. Thereafter, the portion of the main pipe between the stop valves 53 and 54 is filled with reference liquid through a supply port 57. Thus, the zero adjustment of the concentration meter 56 is completed.
As described above, in the conventional concentration meter using microwaves, the measured concentration value may involve an error depending on the mixture ratio of the additives added to the pulp suspension. Moreover, to perform zero-point adjustment, the concentration meter must comprise a bypass pipe and stop valves, and an operation using them are required to fill the concentration meter with reference liquid.